iV.] FOUNDATION OF A THEORY OF HEREDITY. 177 



in the second place the number of ascertained facts appeared 

 to be insufficient to justify a more exact definition. I imagined 

 that the germ-plasm was that part of a germ-cell of which the 

 chemical and physical properties — including the molecular 

 structure — enable the cell to become, under appropriate con- 

 ditions, a new individual of the same species. I therefore 

 believed it to be some such substance as Nageli\ shortly after- 

 wards, called idioplasm, and of which he attempted, in an 

 admirable manner, to give us a clear understanding. Even 

 at that time one might have ventured to suggest that the 

 organized substance of the nucleus is in all probability the 

 bearer of the phenomena of heredity, but it was impossible to 

 speak upon this point with any degree of certainty. O. Hert- 

 wig- and FoP had shown that the process of fertilization is 

 attended by a conjugation of nuclei, and Hertwig had even 

 then distinctly said that fertilization generally depends upon the 

 fusion of two nuclei ; but the possibility of the co-operation 

 of the substance of the two germ-cells could not be excluded, 

 for in all the observed cases the sperm-cell was very small and 

 had the form of a spermatozoon, so that the amount of its cell- 

 body, if there is any, coalescing with the female cell, could 

 not be distinctly seen, nor was it possible to determine the 

 manner in which this coalescence took place. Furthermore, 

 it was for some time very doubtful whether the spermatozoon 

 really contained true nuclear substance, and even in 1879 Fol 

 was forced to the conclusion that these bodies consist of cell- 

 substance alone. In the following year my account of the 

 sperm-cells of Daphnidae followed, and this should have 

 removed every doubt as to the cellular nature of the sperm- 

 cells and as to their possession of an entirely normal nucleus, 

 if only the authorities upon the subject had paid more attention 

 to these statements ^ In the same year (1880) Balfour summed 



1 Nageli, ' Mechanisch-physiologische Theorie der Abstammungslehre.' 

 Miinchen u. Leipzig, 1884. 



2 O. Hertwig, ' Beitrage zur Kenntniss der Bildung, Bcfruchtung und 

 Theilung des thierischen Eies.' Leipzig, 1876. 



2 Fol, ' Recherches sur la fecondation/ etc. Geneve, 1879. 



* Kolliker formerly stated, and has again repeated in his most recent 

 publication, that the spermatozoa (' Samentaden ') are mere nuclei. At 

 the same time he recognizes the existence of sperm-cells in certain 

 species. But proofs of the former assertion ought to be much stronger 

 in order to be sufficient to support so improbable a hypothesis as that 



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